Wolf: Hell no. I like wolves, but they ain't that strong.
Deer: Too much of an herbivore. Not a lot of natural combat strength. I also don't have thumbs. FUCK.
Cat: Cute, but a little small. Sure, it's easy to be picked up as a pet, but self sufficiency is important too.
Tree: I can't move. If someone comes for me, I'm getting chopped down real fast.
A Diety: High potential, but I would probably have to listen to people spout prayers to me and decide who to bless and whatnot. Sounds boring.
Human: Generic. I need some SPICE.
Orc: I might end up reincarnating as an idiot. No. Also, I don't want to end up as a pig-like orc.
Dwarf: I like being tall and not having a beard. No.
Troll: Same thing as the orc, the idiocy problem. Nice regeneration, but fire seems like to obvious of a weakness.
Custom Beastkin: Maybe? Depends on how much flavor you add.
Dragon: Overwhelming power and strength, all at the low, low cost of being a huge monster that can only destroy things. The amount of food necessary to feed yourself would be gigantic! No way am I touching that.
A Saiyan: I refuse on principle. That ain't the kind of fantasy I want to live.
All in all: Custom beastkin is the least terrible option here.